1. Field of the Invention
This invention relates generally to the electrometallurgical melting of titanium and titanium-based alloys. In particular, the invention relates to a method of magnetically-controllable, electroslag melting of titanium and multicomponent high-strength titanium-based alloys, and to an apparatus for carrying out the same.
This invention is useful in the production of titanium and high-alloy titanium alloys characterized by a high density of cast metal, an absence of gas pores and nonmetallic inclusions, and low contents of admixtures. The apparatuses and methods of the invention are particularly useful in the production of special-purpose alloys used for products that operate under conditions of long-term alternating loads, chemically aggressive media, and cryogenic temperatures, e.g., in aviation and shipbuilding industries, power generation and chemistry, nuclear power sector, etc.
2. Description of the Prior Art
Methods for the electroslag melting of metals are discussed in Trochun I. et al., Magnetic Control of Crystallization in the Electroslag Process, Svarochnoe Proizvodstvo, 11: 3-5 (1965) are known.
The above study contains an analysis of the interaction between longitudinally-radial field and electric current, proceeding in a metallurgical pool. It has been demonstrated that such interactions result in bulk electromagnetic forces that affect the melt hydrodynamics and ingot crystallization. However, due to the unidirectional nature of vectors of melting current and the induction of external magnetic field in the course of titanium electrode melting, these forces may cause only an insignificant effect on the hydrodynamics of the melt, and thus exert little influence upon metal purification from admixtures and inclusions, leading to improvement of its macrostructures, microstructures, and quality.
The effect on electric current flowing in slag and metal pools, caused by the radial constituent of external magnetic fields, and resulting in more intense hydrodynamic motion of the melt is discussed in Paton B. E. et al., Development and Studies of Methods of Controlling the Structure of a Crystallizing Electroslag-Produced Ingot by Superposing a Magnetic Field, Problemy Spetsialnoi Elektrometallurgii, 4: 3-7 (1989). However, the melt rotation in the horizontal plane generated by electromagnetic forces, results in the formation of a crater in the central area of a metal pool, leading to the occurrence of a recess in this area, and therefore a negatively affected quality of ingot being melted.
Existing ESR methods are lacking in that they cannot provide metal homogeneity over the total length of the ingot. This is generally caused by the absence of mechanisms aimed at the stabilization of the ingot crystalline structure over the total length thereof by way of stabilizing the hydrodynamic situation in slag and metal melts.
This problem is most critical in the production of ingots made of high-strength, special-purpose alloys used for products that operate under conditions of complicated alternating loads and corrosion. When melting such alloys, it""s highly desirable to differentiate the motion of melt in various areas thereof in terms of direction and intensity. Alloy elements used in these melts comprise heavy metals such as W, Mo, Fe, and Cr which must be uniformly distributed throughout the metal in the course of melting along with light alloy elements such as Al. Therefore, the more intense the melt motion, the more uniform the composition and the more uniform the composition of the crystallized ingot. Here, it is preferable to intensify such stream flows as well as to make the directions of their motion as complicated as possible. This will permit a more complete metallurgical process of dissolution of inclusions in the slag, thereby providing thermodynamic purification of the metal from gaseous admixtures and gas pores carried out by this slag.
The invention provides methods and apparatuses for magnetically-controllable, electroslag melting of titanium and titanium-based alloys wherein, due to the effect on the melting current of at least two opposite radial constituents of the external magnetic field, it would be possible to provide an intense hydrodynamic motion of melt, accompanied by formation of at least two adjoining melt layers rotating in opposite directions. The methods and apparatuses of the invention further provide intralayer and substantially meridional toroidal rotations of the melt, which permit the creation of favorable conditions for improving the homogeneity of the melt""s dynamic composition as well as the metallurgical composition of an ingot.
The methods and apparatuses of the invention also provide the passage of electric current through at least three ring-shaped members in such a way that the current in adjoining ring members would flow in opposite directions. At least 3 ring-shaped members are necessary to provide at least two layers rotating in opposite directions.
The methods and apparatuses of the invention provide one radial constituent which affects the melting current inside the slag pool, and another radial constituent which affects the melting current inside the metal pool, thereby providing rotation of the melts in said pools in opposite directions. The invention also provides a means for affecting the melting current with an external magnetic field. In one embodiment, the melting of an ingot in a fixed crystallizer is provided, while another embodiment provides for the melting of an ingot in the course of lifting a crystallizer.
The invention utilizes spongy titanium, spongy titanium with alloy additives, metallic titanium, or titanium-based alloys as a consumable electrode. The pressure within the melting area is from about 0.9xc3x97105 to about 3.6xc3x97105 Pa, and preferably from about 1.4xc3x97105 to 2.0xc3x97105 Pa.
The methods and apparatuses of the invention provide for the stabilization conditions necessary for the uniform distribution of the flow of a current-carrying fluid, thereby causing a uniform hydrodynamic structure of the melt over the total length of the ingot, preferably while maintaining constant the melting current, the feed rate of consumable electrode, and the electrode gap. The methods and apparatuses of the invention provide optimum conditions for running the processes, in which the melting current and feed rate of the consumable electrode results in the consumable electrode melting in the upper portion of the slag pool with a maximum permissible value of the electrode gap. Furthermore, conditions for the invention provide optimum processes by smoothly decreasing the melting voltage, and by providing apparatuses for carrying out such a mode.
The methods of magnetically-controllable, electroslag melting of titanium and titanium-based alloys comprise the steps of:
providing a consumable electrode in electrical contact with a crystallizer filled with a metered amount of flux;
evacuating a crystallizer melting area and supplying an inert gas thereto;
passing an electric current through said electrode, causing the melting of flux and the consumable electrode and resulting in the production of a melt of slag and metal pools;
affecting said melting current with an external magnetic field having at least two opposite radial constituents disposed in parallel planes, thereby resulting in the formation, within the melt bulk, of at least two adjoining melt layers rotating in opposite directions, as well as intralayer and substantially meridional toroidal rotations of the melt;
crystallizing a metal ingot at the interface with said metal pool as said metal pool is replenished through the melting of the consumable electrode;
withdrawing said ingot from said crystallizer.
Here, it is preferable to generate the radial constituents by passing an electric current through at least three ring-shaped conductor members surrounding the crystallizer. The ring-shaped conductor members are spaced at equal distances not exceeding half total depth of the slag and metal pools. The electric current in adjacent ring members is passed in mutually opposite directions.
In one embodiment of the invention, the melting current is affected by an external magnetic field. More specifically, one of the radial constituents affects the slag pool current, while another opposite radial constituent affects the metal pool current, thereby causing the rotation of the melts in mutually opposite directions.
Preferably, the consumable electrode comprises a material selected from the group consisting of spongy titanium, spongy titanium with alloying additives, titanium, and titanium-based alloys. The inert gas pressure is preferably from about 0.9xc3x97105 to about 3.6xc3x97105 Pa, and preferably from about 1.4xc3x97105 to about 2.0xc3x97105 Pa.
In another embodiment of the invention, the conditions for uniform distribution of the flows of current-carrying liquid are stabilized, resulting in a substantially uniform hydrodynamic structure of melt over the total ingot length. Stabilization of these conditions are carried out by maintaining the melting current, consumable electrode feed rate, and electrode gap at substantially constant levels. The constant levels of melting current and consumable electrode feed rate are preferably selected to provide for the melting of the consumable electrode in the upper portion of the slag pool with a maximum permissible value of the electrode gap.
In another embodiment of the invention, the melting voltage is smoothly decreased, thus maintaining the level of melting current with maximum permissible electrode gap and feed rate of the consumable electrode substantially constant.
The objects of the invention are also achieved by providing an apparatus for magnetically-controllable, electroslag melting of titanium and titanium-based alloys, comprising:
a crystallizer provided with an internal volume forming the melting area partially filled with a metered amount of flux;
a vacuum chamber interfacing with said crystallizer melting area;
means for supplying said crystallizer with inert gas for developing gauge pressure therewithin;
means for positioning said consumable electrode inside said melting area in electrical contact with said crystallizer;
a power supply providing current passage through said consumable electrode and said flux, and subsequent formation of slag and metal pools respectively as the electrode gets melted;
means for affecting the melting current by way of an external magnetic field having at least two opposite radial constituents disposed in parallel planes, thereby resulting in formation, within the melt volume, of at least two adjoining melt layers rotating horizontally in mutually opposite directions, and in intralayer and substantially meridional toroidal rotations;
means for carrying out relative travel of an ingot being melted and said crystallizer in the course of crystallization, and for removing a resulting ingot from said crystallizer.
The means for affecting the melting current by way of an external magnetic field preferably comprises at least three ring-shaped conductor members secured to the crystallizer external surface in parallel relationship, and connected so that electric current in adjoining ring members flows in opposite directions. The ring-shaped conductor members are preferably secured to the crystallizer external surface covering the slag and metal pools, and spaced at distances not exceeding half the total depth of the slag and metal pools, with the crystallizer being provided for traveling with respect to an ingot being melted.
In one embodiment, the ring-shaped conductor members are secured to the crystallizer external surface covering the slag and metal pools, and spaced at distances not exceeding half the total depth of said slag and metal pools, with the crystallizer being fixed with respect to an ingot being melted.
In yet another embodiment of the inventive apparatus, a means for stabilizing the melting current is provided, thus resulting in substantially constant levels of the melting current in the mode of maximum permissible electrode gap at a substantially constant feed rate of the consumable electrode.
The apparatuses of the invention are preferably connected to a transformer which can be connected to any known power supply. Furthermore, the means for stabilizing the melting current preferably utilizes a thyristor controller and a control unit connected into a circuit unit of the primary winding of the transformer.